This invention relates to a novel system for the reforming of a laser beam having a circular sector shaped beam cross section by means of a mirror having a reflective surface configured as the circular sector of a rotational body.
The laser beam forming system described in DE 44 21 600 C2 converts a circular sector profiled laser beam with radial and/or azimuthal polarization into a rectangular laser beam with linear polarization. To achieve that effect, the laser beam is shaped by means of a cone sector mirror and a parabolic cylinder mirror, and the line focus of the parabolic cylinder mirror approximately coincides with the axis of rotation of the cone sector mirror. However, this prior art beam forming system requires the use of two mirrors. The cone sector mirror shapes the laser beam in the azimuthal direction; however, for the focussing or defocussing of the laser beam in the radial direction, an additional mirror must be employed. The focal lengths and beam dimensions involved are of an order of magnitude at which, in the case of spherical mirrors, spherical aberrations lead to image distortions.
To permit decoupling of the laser beam from the laser resonator, it is necessary in the case of the prior art beam forming system employing a cone sector mirror for the cone beam angle to deviate slightly deviate from 90xc2x0 or for the cone axis to deviate somewhat from a precise coincidence with the line focus of the parabolic cylinder mirror. As a result, only approximate rectangularity of the beam cross section and linearity of polarization can be obtained.
Accordingly, it is an objective of the present invention to provide a novel beam reforming system which minimizes image aberrations.
It is also an object to provide a laser including such a laser beam reforming system in which the laser beam is linearly polarized.
It has now been found that the foregoing objectives may be accomplished by a beam reforming system in which the reflective surface of the mirror is configured as a circular sector of a parabolic rotational body. This reflective surface may be either the convex or the concave surface of a parabolic rotational body, and the parabolic rotational body is preferably in the form of a rotational paraboloid.
A parabolic rotational body is produced by rotating the parabola z=x2/a2 less than 0 around any given axis of rotation that extends parallel to the axis of symmetry of the parabola. The curvature of the parabola is d2z/dx2=2/a2, and, for a2 greater than 0, it differs in all cases from zero. A circular sector of a parabolic rotational body could also be approximated by an elliptical rotational body. A rotational paraboloid constitutes a special parabolic rotational body in which the axis of rotation coincides with the axis of symmetry of the parabola.
Reflective surfaces in the form of circular sectors of parabolic rotational bodies shape the laser beam both in the azimuthal and in the radial direction. With mirrors of this type, no image aberrations whatsoever are encountered, and if there is precise parallelism between the axis of the laser beam and the optical axis, there will also be no astigmatic distortions. The circular sector of a parabolic rotational body produces a line focus on its axis of rotation, whereas the circular sector of a rotational paraboloid produces a point focus on its axis of rotation.
When the circular sector of the parabolic rotational body is in a coaxial position with the circular sector axis of the incident laser beam and the circular sector shaped laser beam is reflected at the parabolic rotational body by 90xc2x0, the radial and/or azimuthal polarization transitions into a linear polarization. The effective focal length of the circular sector of the parabolic rotational body corresponds in the azimuthal direction to the radius of curvature of the circular-sector-shaped laser beam. Reflection by the parabolic rotational body or by the rotational paraboloid reforms the circular sector shaped laser beam into a rectangular laser beam while the parabolic rotational body mirror produces on its axis of rotation a line focus and the rotational paraboloid mirror produces on its axis of rotation a point focus. Since the optical axis extends parallel to the axis of the laser beam, no astigmatic distortions are generated and image aberrations as a whole are minimized.
It is desirable to provide an aperture (spatial filter) in the line focus of the circular sector of the parabolic rotational body or in the point focus of the circular sector of the rotational paraboloid to filter undesirable diffraction components (secondary maxima) from the rectangular laser beam. The relative position between the filter and the line or point focus of the circular sector may be adjustable. A measuring device can be used to detect the temperature at the filter and to generate a corresponding temperature signal which serves as a control signal for adjusting the adaptive mirror or for moving the aperture.
In preferred embodiments of this invention, an optical element with one or several surfaces is positioned downstream from the circular sector and its surface(s) serves to modify the laser beam in each case so as to extend in two mutually perpendicular directions. This optical element makes it possible at the point of application to form the desired width of the laser beam.
In a variation of this design, the optical element is configured as a single unit bifocal lens whose rearward focus is preferably positioned in the area of the axis of rotation of the parabolic rotational body.
In another embodiment, the optical element is configured as a multi-component device, consisting for instance of a cylindrical lens and a parabolic cylindrical mirror, or of a cylindrical lens and two parabolic cylindrical mirrors. The rearward line focus of the cylindrical lens and/or the rearward line focus of the parabolic cylindrical mirror is/are preferably positioned in the area of the axis of rotation of the parabolic rotational body. The cylindrical lens may serve both as a vacuum seal on the laser resonator and as an output window. A parabolic cylindrical mirror may also be approximated by an elliptic-cylindrical mirror.
The circular sector of the parabolic rotational body and the cylindrical lens can jointly constitute a Galilean telescope for what was the radial and is now the deradiused direction of the circular sector shaped laser beam. The circular sector of the parabolic rotational body jointly with a parabolic cylindrical mirror can constitute a Kepler telescope for what was the azimuthal and is now the deradiused direction, and the two parabolic cylindrical mirrors can jointly constitute a Kepler telescope in the original azimuthal direction.
This invention also applies to a laser having a coaxial laser resonator with an annular discharge chamber and a circular sector shaped decoupling, i.e. output opening as well as a system for beam reforming as described above. In this case, the circular sector of the parabolic rotational body is coaxially aligned with the circular sector axis of the incident laser beam, i.e., the axis of rotation of the parabolic rotational body coincides with the circular sector axis of the incident laser beam. This reforms the circular sector shaped laser beam with radial and/or azimuthal polarization into a rectangular laser beam with linear polarization.